Jaycar's dirt cheap QM-1502 mini DMM

[manuka]

Perhaps of most interest to Kiwi/Aust. readers? Jaycar have new QM-1502 mini DMMs back in stock,at a throw away ~Aust$5. It's hard to know if one should laugh or cry at such trivial prices -I recall (as an early teen) paying ~$500 in todays money for a far inferior meter...

The QM-1502 build is suprisingly good, & current drain an astoundingly low ¼ mA from the A23 12V battery. Aside from their skinflint & toolbox spare appeal, they also suit use as an inbuilt panel meter. But with no auto power off the prospect of flat batteries limits their schools use. Educators who've found their class meter batteries dead mere minutes before 30 surly youths arrive for a lab session will appreciate this particular "electro-angst"...

Right then Forum Brains Trust, the challenge is out to suggest a COST EFFECTIVE autopower off circuit to enhance them.

 

[Jeremy Harris]

I bought a couple of similar dirt cheap DMMs from ebay a couple of years ago, as "disposable" ones I could use on the boat. The idea was that I wouldn't get too upset at losing one overboard or by it getting water damaged. They've proved to be remarkably accurate and tough so far, both are still working well. Like the one you've mentioned, they don't have an auto off, but although I've accidentally left them turned on overnight a few times now they are both still on their original batteries, two years or so on. Mine use PP3 batteries, and IIRC, I almost paid more for the batteries than I did for the meters (mine came minus batteries, but the meters were only something like £2.50 each, including postage from China!).

The leads they come with are pretty rubbish, I found, and although the meter has a 10A current scale the supplied leads are unhappy at this sort of current. Not a significant problem for most stuff, as I can always use the leads from the "proper" DMM if need be.

 

[manuka]

Jeremy: Agreed on the lite leads! Banana plugs suit DIY versions however. I've had similar disposable DMMs suprising last for some years too, & always keep one in my car tool kit- I just saved myself replacement of a costly lamp by instead identifying a dirty connector as the "no go" culprit.

Google suggest this Darlington based PNP autopower off circuit, & of course Maxim make specific ICs for such tasks. A further benefit of an auto power off is that it can prolong a DMM's rotary switches life. If an external switch is instead fitted then that does the switching, which also allows a preselected range to remain in use at power up.

 

[Buzby]

Does it still pull < 0.25mA when testing transistors ?

 

[manuka]

Not checked yet (as none handy & likely to be only a very occasional function anyway), but it "surges" to a blistering 0.4 mA on resistance when the leads are shorted! Note - supply doesn't seem critical, as the DMM runs a treat from 9V & the "low batt" function is only displayed at ~7V

 

[boriz]

Picaxe 08 + push-to-break button. Picaxe can low-side switch the power with a small FET. Low power sleep after (say) 20 minutes, push button to restart. Doesn't get much simpler/easier.

 

[Buzby]

Picaxe 08 + push-to-break button...

Could maybe get away with no switch, just pick something off the back of the rotary.

With current demand as low as it is, could the 08 supply the low side directly by switching a pin between 'Low' and 'Input' ?.



Completely untested !.

 

[eclectic]

In post #1, Stan capitalised
"COST EFFECTIVE"

Just using this NZ example:

http://www.sicom.co.nz/?PageID=3617&function=viewprd&rnd=4803018&prd_id=337359&pmm_id=0

the raw cost of the PICAXE plus a few components is similar
to the cost of the Jaycay multimeter.

A genuine "don't know", but, in a classroom situation,
how about designating a

"trusty" to turn all the meters off?

e

 

[Buzby]

I was thinking SMD :

http://www.techsupplies.co.uk/epages/Store.sf/en_GB/?ObjectPath=/Shops/Store.TechSupplies/Products/AXE007M2SM

But I see your point. How do you judge 'cost effective' when the kit costs less than the battery ?

Maybe instead of trying to turn it off, we can find a way to keep it on !

Solar power ?

EDIT : Half baked idea alert !!!

Fit a USB socket, and when the meter goes flat it's the kid's job to recharge it from his mobile phone.

EDIT 2 : Not so half baked.

Put a big cap across the meter supply, and use a push-to-make to charge it from the battery.

It should run for a good while before the 'low bat' warning shows.

 

[boriz]

Addition to the great cap idea: FET + CAP Will stay powered until the cap self-discharges. Will last longer and can use smaller cap.

 

[manuka]

As Q = I x t = V x C, then a ¼ mA drain at 12V should hold up (until "low battery") for a good minute with a 2200 -4700 uF electrolytic capacitor. This was nicely confirmed OK, & could suit push switch operation for quick checks on perhaps circuit charge/discharge currents or supply voltages. Recharging cost effectively may however be a problem

It's tempting perhaps to grab a few tiny PVs off cheapo calculators- most provide ~3mA at several volts. The likes of Pager Motors offer small PVs that may better suit.

 

[erco]

Harbor Freight, a cheap tool chain in the US, sells those for ~$4, and even gives them away occasionally for free:
http://moneysavingmom.com/2011/09/harbor-freight-free-socket-set-digital-multimeter-or-power-strip-with-purchase-today-only.html

Some even have a backlight! They work fine for 98% of my needs, and it's hard to kill them, other than blowing a fuse. Great little meters, I have several and rarely use my fancy meters anymore.

12V batteries? Ours use rectangular 9V batteries, they still last for several days if you leave them on.

 

[SAborn]

I think electric shock treatment to the kid that left the meter on would work, nothing like education 101, even dumb animals learn like this.
Only problem is i think shock treatment and floggings of students are no longer permitted in NZ........pitty.

The FET + Cap idea might be the simplest, with less bruising or burn marks

 

[Pongo]

Harbor Freight, a cheap tool chain in the US, sells those for ~$4

The Harbor Freight ones are hulking big things compared to these tiny Jaycar ones.

 

[erco]

Surely these dimensions from the website can't be correct? Sheesh! I'd lose that on my bench...

Dimensions 56.5(H) x 30.3(W) x 24(D)mm

 

[Pongo]

See the comparison pic with a 9 volt battery in the first post, that battery is about 50 by 25 mm, so I would say not.

 

[manuka]

Jaycar's QM-1502 mini DMM is indeed tiny,but dimensions of one to hand actually are ~45mm(W) x 93mm(H) x 25mm(D). It's hence about the footprint of end on "4 by 2" (2 inches x 4 inches) dressed framing timber. A comparison pix is attached for your viewing pleasure. Stan.

 

[premelec]

@Stan - are those crimps in the thick wire at the bottom [post #1] of the picture a method of calibrating the 10A shunt [and possibly using it as a fuse too...] ? We used to file resistors to change value but I don't think I've seen diagonal cutters used to modify a wire's resistance before - if that's the situation....

 

[westaust55]

The Jaycar mini DMM is stated as CAT II 250 Volt so seemingly good.
However for many small cheap and pocket style DMM if there is any intent to use to measure other than extra low voltages (ie <32 Vac or 50 Vdc) one should prior to purchase ensure the CAT rating and that the input voltage range is suited to the voltages to be measured. Is 250 V average, rme or peak ?
Keep in mind that 240 V rms has a peak to peak of around 670 volts and thus a DMM for a typical 230/240 V household mains single phase supply the meter needs to be rated to handle. Peak to peak voltage of at least 670 V and preferable 700 Vp-p.

 

[erco]

Looks like Jaycar also sells the same larger model as HF: http://www.jaycar.com.au/productView.asp?ID=QM1500

The smaller one is pretty dang cute, though.

 

[boriz]

http://www.ebay.co.uk/itm/DT830B-LCD-Digital-Multimeter-Voltmet-Electric-Voltage-Tester-BATTERY-LEADS-/280955139483?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item416a3b359b

Wouldn't trust it for mains, but can't beat the price for low voltage hobby stuff.

 

[manuka]

FWIW-motivated by a nearby brightspark youngster, & with an Instructable in mind,I rustled this up last year as an inital DMM enhancement.

Extra: This Celsius readout LM35 based temperature hack also caught my eye,although it'd perhaps better suit a flying lead probe (rather than inbuilt). National Semiconductor's TO-92 packaged LM35 (Celsius) & LM34 (Fahrenheit) are easy to use precision analog temperature sensors,& draw ~60µA in use. However neither are cheap (~$6 Digikey). Classic semiconductor diode approaches,or "diode-coupled" transistors (with base & collector shorted) may tempt skinflints. Stan.

 

[fernando_g]

@Stan - are those crimps in the thick wire at the bottom [post #1] of the picture a method of calibrating the 10A shunt [and possibly using it as a fuse too...] ? We used to file resistors to change value but I don't think I've seen diagonal cutters used to modify a wire's resistance before - if that's the situation....

Good catch... most likely you are correct, it is a "shunt calibrator". And they had to do it in several places, otherwise if you cut to deep in a single place, the reduced cross section would fuse at 10 amps.

I also filed resistors to "adjust" its resistance. They had to be carbon composition, though, with a solid core. 1 watters were the easiest to file.

I was once told by an old timer (even older than myself), that radio and TV sets in the old days had redundant resistors or capacitors in parallel. To adjust them, one would trim out one by one until the correct adjustment had been reached. He claimed that it is where the electronics slang-word "trimming" comes from.

Back to the original post.....there is NO EXCUSE anymore, for anyone that troubleshoots or plays with electricity/electronics for not owning a meter.
I'm still surprised that some posters in this forum and others forums that I follow, when asked if they own a DMM, the reply nooooooooo!

 

[premelec]

Hi fernando... we used to trim up capacitance [condensers ] with 'gimmicks' a twisted pair of wire which you clipped shorter to trim down to required value... if you really want to you can modify some metal film R with sand blasting [or the more proper laser trimming...].... I notice the DMM Boriz listed above has quite a shipping fee! [appears to be a listing error... ]. I haven't tried the pinch trim method yet - should be on constantan or other zero tempco wire as it's bound to heat up at 10 amps... Whatever works!

 

[srnet]

Back to the original post.....there is NO EXCUSE anymore, for anyone that troubleshoots or plays with electricity/electronics for not owning a meter.

Or to not have a pencil and paper (remember them ?) handy to draw a circuit diagram.

 

[fernando_g]

I had forgotten about the "gimmick" caps....

You may also remember the air-wound coils used in VHF and UHF work. With a ceramic or wooden tool, one would spread out the windings while watching a spectrum analyzer, to obtain the proper peaking or notching at frequencies marked with "birdies". After the tuning was complete, they would pour some molten wax to hold the shape of the distorted coil in place.

 

[premelec]

Ah the smell of hot wax - from wax sealed capacitors as well as for holding coils & tuning slugs [these days you just find hot melt glue]! Nostalgia time and I still find hot solder flux a sort of perfume [BTW some solder actually was perfumed... for the ladies on the assembly lines I presume...] . I still don't like overheated epoxy or phenolics... stink...